The invention relates to an optically readable information disc which comprises a substrate plate in the form of a disc manufactured from a synthetic resin and which comprises on one or on both sides a reflecting optical structure which is read by means of laser light via the substrate plate.
Such a disc is commercially available. An example is the video disc or laser vision disc which comprises optically readable video information. Another known example is the audio disc or compact disc which comprises optically readable audio information. Both in the video disc and the audio disc the reflecting optical structure comprises a usually spiral-shaped optically readable information track of information areas situated alternately at a higher level and at a lower level. The difference in height between the areas is 0.1-0.2 .mu.m. The longitudinal dimensions vary in accordance with the stored information and are from a few tenths of a micron to a few microns. On the other side of the information track the disc is covered by a reflection layer, for example, a layer of Ag or Al. The video disc has comparatively large diametrical dimensions of, for example, 30 cm. The audio disc usually is considerably smaller and has a diameter, for example, of 10 cm.
A third type is the so-called recording disc. The reflecting optical structure comprises a recording layer, for example, a layer of dye, a layer of metal, for example Bi, a layer of a semiconductor element, for example Si, or a layer of chalcogenide, for example, a Te-Se alloy. By exposure to pulsated laser light, information bits are formed in recording layer which consists, for example, of holes or cavities in the recording layer, de-coloured spots or amorphous spots in a crystalline substrate, or conversely. The recording disc may comprise a follower track or servo track which may comprise optically readable information. The recorded information bits are read optically by means of laser light.
In applicants' opinion the quality of the substrate plate is of great importance for the quality and stability of the optically readable information disc. Various requirements have to be imposed upon the substrate plate. First of all the substrate plate must be transparent to the laser light used. The plate may show only a small birefringence. Internal stresses in the substrate plate which may be the result of the manufacturing process, for example, an injection moulding process or a compression moulding process, lead to an increased birefringence and must consequently be avoided. Another important factor is the non-defromability. In applicants' opinion two aspects play a role, namely the extent to which moisture is absorbed and again desorbed by the substrate material, as well as the shrinkage behaviour of the material. The shrinkage behaviour is the extent of plastic deformation under the influence of mechanical load and/or internal stresses. The glass transition temperature (devitrification temperature) of the material is an important parameter for the shrinkage behaviour. At a high glass transition temperature the shrinkage behaviour is favorable and the said plastic deformation is small.
So far a substrate plate of polymethylmethacrylate (PMMA) has been used for optically readable information discs having a comparitively large diameter, for example, the video disc. PMMA has a good optical quality with little or no birefringence. A disadvantage is that said disc is warped by the one-sided moisture absorption at the surface of the PMMA substrate plate remote from the optical structure. In order to mitigate this, or rather to compensate for said warping effect, two information discs are adhered together, the optical structures facing each other.
For optically readable information discs having a small diameter, for example, the audio disc, a substrate plate of polycarbonate is used. Polycarbonate, compared with PMMA, has the big advantage of a low moisture absorption. The optical quality of polycarbonate is good. The stronger birefringence of polycarbonate as compared with PMMA is admissible. Unfortunately, polycarbonate cannot be used in information carriers having comparatively large dimensions, for example the video disc. The manufacture of such discs by means of injection moulding requires such process conditions in regard to temperature of the polycarbonate and of the propellant screw and matrix, injection speed profile and pressure variations, that an unacceptable. birefringence is the result.